New visible-light photocatalysts were prepared by doping In2O3 with nitrogen from ethylenediamine (NH2(CH2)2NH2) or ammonium chloride (NH4Cl) as the nitrogen source. Nitrogen-doped In2O3 powder prepared from NH2(CH2)2NH2 shows a rhombic structure and a substitutional N-doping, while the powder prepared from NH4Cl shows a cubic structure and NH x in interstitial sites. N-doping extended the light absorption of In2O3 to the visible region (λ < ∼650 nm), narrowing the band gap from 3.5 eV to approximately 2.0 eV. The photocurrent densities of N-doped In2O3 electrodes are at least double those of undoped In2O3 and approximately 50 times better than N-doped TiO2 electrodes in the visible region, although optimization will be needed to deliver high photocurrents. This present work shows that In2O3 can be suitably doped to produce a promising photocatalyst with improved photoelectrochemical properties for solar hydrogen conversion applications.

The solid-state nuclear magnetic resonance (SSNMR) analysis and structural characterization of N-doped TiO2 nanoparticle and monolayer materials suitable for visible photocatalysis is reported. The SSNMR analysis of 15N-doped TiO2 synthesized using 15N-urea before calcination indicates formation of various amino functionalities of the type NH, NH2, NH3, and probably NH4 +, while the NMR spectrum of the yellow powder that results from high-temperature calcination shows that these nitrogen species oxidize to form nitrate. These and nonisotopically labeled materials were characterized also by X-ray diffraction, UV−vis, and electron paramagnetic resonance spectroscopy. Monitoring the oxidation of 1,2-13C-trichloroethylene (TCE), photochemical activity was confirmed by monitoring the production of CO2 by 13C NMR analysis. The powdered form of N-doped TiO2 proved to be a highly efficient visible light catalyst by oxidizing all of the TCE to chlorinated alkyl and acyl containing molecules along with CO2. The performance of a TiO2−N−TiO2/porous Vycor 7930 borosilicate glass monolayer catalyst was in turn evaluated by monitoring the photochemical oxidation of ethanol and acetone with UV−vis light, and it was found to perform better than TiO2-only monolayers. Direct nitridation of TiO2 powders and monolayers also was conducted to compare the 15N SSNMR and to discern if there is a nitridic bond in these materials. The SSNMR results provide more evidence supporting the hypothesis that the nitrogen atom in N-doped TiO2 is present in interstitial sites when N is in a highly oxidized state.

N-doped In2O3 films and powders were synthesized, characterized, and evaluated for photoelectrochemical water splitting. The synthetic process was followed in detail by FTIR and UV−vis spectroscopy and the In complex was characterized by X-ray crystallography. NMR, XPS, and EPR were combined in an effort to track the N speciation at each step of the synthesis. The structural, optical and photoelectrochemical properties of the final products (films and powders) were analyzed. Compared to undoped In2O3, N-doped In2O3 showed an increased absorption in the 350−500 nm range with a red shift in the band gap transition. Electrodes prepared from NH4Cl exhibit higher photoactivity compared to the electrodes prepared from urea. NMR, XPS, and EPR results showed that inert amino- and nitrate-type species adsorbed on the surface were produced from urea and NH4Cl, which count toward the N atomic percent but do not increase the activity of In2O3. However, a nitrate-type species in interstitial sites and a paramagnetic species attributed to an F-center play an important role in the photoelectrochemical improvement of N-doped In2O3 prepared using NH4Cl as the dopant source. A mechanism for the formation of the F-centers is proposed based on the electron donation of the nitrate-type species (NO x −) at oxygen vacancies. N-doped In2O3 prepared using NH4Cl at an optimal N content of 1 to 2% (initial N/In = 0.50) produced 5 fold better photocurrent density than undoped In2O3, reaching close to 1 mA/cm2 with a film thickness of 15 μm and applied voltage of 0.7 V. This paper also illustrates how the combination of NMR, XPS, and EPR has excellent potential for characterizing dopant species and for determining the origin of visible photoelectrochemical activity of doped metal oxides.

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